Hi there, Natalie,
Welcome to planDisney. Whether you are planning your first Disney cruise or your hundredth, we are happy to offer our support and advice to help make your experience as enjoyable as possible.
While there are firm limits to how much alcohol each eligible Guest may bring onboard a Disney Cruise Line ship, there are no similar limits to the amount of water or soft drinks you may carry on. Beverages should be packed in your carry-on luggage. So, as a matter of practicality, the limit is as much as you are willing and able to carry with you. Note that carry-on luggage must be small enough to fit through a typical airport-style carry-on luggage scanner. Also, coolers are not permitted, unless they are for medications, baby food, and/or related to dietary restrictions.
Besides all the water and other beverages you carry on, you may also enjoy a wide variety of beverages onboard, many at no extra charge, including soft drinks, tea, and regular drip coffee served during meals and available from the beverage station or the Vibe teen club. I also like to bring a refillable water bottle to fill up at the multiple water refill stations located throughout the ship.
Tony's Tip: I sometimes pack small, lightweight, flavored drink mixes to use with my refillable water bottle. To me, they are great for when I want a simple, low-calorie drink, but without having to carry a large amount of bottled liquid onboard.
I'm glad you are planning on staying hydrated during your Disney cruise, Natalie. You'll want to be healthy and well to enjoy all of the amazing entertainment, characters, dining, and so much more to experience on your cruise.
Have a magical time, Natalie!
~Tony